feat: re-expose 'http-credentials' from 's3'

[woshilapin]

aws-creds is re-exposed from s3. This means, technically, we don't need to depend on aws-creds when we already depend on s3. However, if we need the http-credentials from aws-creds, then we do need to depend on both which can create problems:

• aws-creds gets updated, but s3 didn't yet get updated with the new version of aws-creds
• our code uses Credential from the new version of aws-creds and try to use it with s3 which depends on the old version of aws-creds
• you get an error

That's usually the reason for re-exposing aws-creds from s3 (to avoid this different versions of the same crate in the dependency tree). But today, there is no way to have http-credentials without importing both. This PR should solve that.

[durch]

@woshilapin, good call, merged, published in 0.34.0-rc4